The invention relates to a manner of operating extraction apparatus or extractive distillation apparatus or a combination of both apparatus for the purpose of recovery of pure hydrocarbons.
Installations of this type are presently used on a large industrial scale, particularly where it is desired to obtain pure aromatic compounds, for instance from a mixture of aromatic and non-aromatics. In these processes the selective solvent is preferably taken from the group of the N-acyl derivatives of morpholine, specifically N-formylmorpholine. Normally, the extraction is in the manner of a liquid-liquid extraction where the solvent is employed together with water. In the so-called extractive distillation, however, the operation is usually carried out with anhydrous solvent. A prior art process described in German Pat. No. 2,040,025 operates with a combination of liquid-liquid extraction and extractive distillation. Reference regarding this type of combination apparatus is also made to U.S. Pat. No. 3,554,873.
In the apparatus used in these processes there were noticed in a number of cases corrosion phenomena for which at first there was no plausible explanation. The corrosion appeared particularly at those parts of the apparatus which were exposed to strong attacks by vapors and liquids as for instance is the case with the boiler pipes at the bottom of the distillation column or of the extractive distillation column. Among the experts it was generally believed, as for instance expressed in the German published application 1,545,365, column 2, lines 35 to 43, that the corrosion was caused by hydrolysis or disociation products resulting from the morpholine derivatives used as solvents. Careful investigations of the applicants have however not confirmed this assumption.
An attempt was also made to solve the problem by employing methods used in gas washing apparatus for the purpose of preventing corrosion. In these apparatus the amino solutions of tartaric acid were used in combination with compounds of the 4th and 5th group of the periodic system. However, this method also did not have a satisfactory result.
The present invention therefore has the object to provide for means to prevent the corrosion phenomena set out above. Implied in this object is also the problem to find the cause of the corrosion which in the past was entirely unknown.